1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to a variable camshaft timing device with hydraulic lock in an intermediate position.
2. Description of Related Art
U.S. Pat. Nos. 6,814,038 and 6,941,913 disclose a variable cam timing system that utilizes the same spool that controls the VCT system to actively control the lock pin. The positions of the spoors lands directly influence whether source oil is supplied to both the lock pin and either the retard or the advance chamber of the phaser.
U.S. Pat. No. 6,666,181, which is hereby incorporated by reference, discloses a variable cam timing device which can be set to default in an intermediate phase angle position located between the advance and retard mechanical stops. More specifically, a hydraulic detent circuit is actuated via a control valve to command the variable cam timing (VCT) device to a position somewhere in the middle of the total phase angle range of authority.
The two features of a spool controlling the lock pin and a hydraulic detent circuit actuated via a control valve to command the VCT to a position somewhere in the middle of the total phase angle range of authority can be combined on one VCT assembly to be controlled by the spool valve, but it is not practical to do so. The problem with this approach is that there would be three hydraulic circuits on one spool valve, one to control the VCT, one to control the hydraulic detent circuit that commands the VCT to a known intermediate position and one to control the lock pin. This makes the spool valve and sleeve very long, making them very difficult to manufacture. In addition, putting all three hydraulic circuits on the control valve increases the overall package length of the VCT, which is not well received in the tight package requirements of the automotive powertrains. Finally, putting all three control circuits on one spool valve makes for complex and restrictive flow circuits, thus limiting the performance of each circuit.
GB 2437305 teaches different embodiments in which one or two locking pin are used with either a double acting spring or a hydraulic circuit under the action of cam torque reversals to return the phaser to a locked position.
In one embodiment, two one-way valves within the phaser allow oil to escape the chambers in response to torque in one direction or the other. The bores of the lock pins are each connected to one-way valves by an oil drilling that also enters the adjacent cavity formed between the housing the rotor in which the vane is present. When the phaser is unlocked and oil pressure drops, one lock pin locks the rotor relative to the housing and the other runs against the surface of the end plate. When the lock pin is locked, oil can flow through the drilling and pass through to a one-way valve to the adjacent cavity to move the phaser to a position where the second lock pin can engage and lock. If a lock pin is unlocked, the diameter of the lock pin prevents fluid from flowing to the one-way valve. This system is under passive control. In other words, another valve does not directly influence the fluid that acts on the lock pins.
In another embodiment, two one-way valves are present in the phaser and are connected to a single lock pin. A third drilling leads into the locking pin bore and this hole leads through a thin manifold plate into a slot in the front plate of the phase. The slot acts to connect the first hole to the other two holes in the manifold plate that are selectively covered and uncovered by one of the vanes. In the locked position, the vane obscures both holes. Any movement of the phase away from the locked position, allows oil to flow out of the associated cavity under the action of cam torque reversals and into the opposing set of cavities via the one-way valve. When a one-way valve is connected to the cavity, the other one-way valve is connected to the bore of the single lock pin. When the lock pin is locked, oil feed to both one-way valves is obscured to both one-way valves. When the lock pin is unlocked, oil feeds connected to the reduced diameter of the lock pin. This system is also a passive control system. In other words, a valve within the phaser or remotely does not directly influence the pressure acting on the lock pin to move it to a locked or an unlocked position.
Therefore, there is a need for a simple way of positioning the phaser in an intermediate phase angle position using an actively controlled detent piloted valve, while keeping the overall package length the same or smaller and increasing performance of the VCT phaser.